Well construction using small laterals

ABSTRACT

This invention relates to the construction of well such as oil and gas wells using techniques based on drilling small lateral wells from a main well. The problem of narrow pressure window is solved by the use of constructions techniques that are based on the use of lateral boreholes, i.e. secondary boreholes that are drilled a main borehole, wherein a method of constructing a well comprises drilling a main borehole extending from the surface through one or more underground formations, drilling a plurality of lateral boreholes extending from the main borehole into surrounding formations, wherein the lateral boreholes are substantially shorter and of smaller diameter than the main borehole; and wherein each lateral borehole is separated from its neighbouring lateral boreholes by a relatively short distance. Drilling of the lateral boreholes can be for to extend 5-60 metres from the main borehole with a diameter in the range 3.8-10 cm by the lateral boreholes at an axial spacing of less than a few metres in the main borehole. Drilling of more than one lateral borehole can be done at the same depth in the main borehole with a trajectory that deviates from the main borehole by less than 10° or with trajectories that extend in a plane that does not contain the main borehole.

TECHNICAL FIELD

This invention relates to the construction of well such as oil and gaswells using techniques based on drilling small lateral wells from a mainwell.

BACKGROUND ART

Well construction has a number of well-known problems that can affectthe ability to recover oil from the formation through which the well isdrilled, or even, in extreme, circumstances to complete the well andbring it to production.

When drilling horizontal wells in oil-bearing formations, a key factorfor success is to try to keep the well a constant distance above thewater table that underlies the oil. When this is not achieved and thewell trajectory varies, the low points or ‘valleys’ of the well areoften sources of problem. If open-hole completion is used or if theperforation density in that region is uniform, there is a high risk ofwater coning towards the well valley. Even with cased hole and noperforation in the valleys, some lengths of well can still lose contactwith the reservoir. In rare situations, a drilled section of well mustbe abandoned and side-tracking is performed to re-position the well atthe correct depth.

A similar problem may appear in the “up-hill” part of the well when thedistance from the well to gas layer overlying the oil is too small. Inthis case gas can then be produced, with similar consequence andtreatment as the “valley” and water problem described above.

In some wells, the drilling process itself generates some formationdamage in the near well-bore region. This appears as a high skin effectwith a consequential production limitation. Certain chemical treatmentshave previously been proposed to be performed in the rock matrix forcleaning the rock pores and re-establishing the proper permeability butthese are not always effective.

For sand control during production, the common solution is to use gravelpacking and screens. In horizontal wells, the placement of the gravelcan be quite challenging, while at the same time reducing the flowsection (the open bore of the well) in the completion. For completionsbased on gravel packing and fracturing (“Pack&Frac” technique), therecan be difficulties with the placement of the pack, and there is nocontrol of the direction of the short fractures produced.

For the problem of formation collapse due to stresses in the rock (andstress concentration near the well bore), the only solutions are eitherto adapt mud density used during the drilling of the well with risk offracturing other layers, or to abandon this section of the well andrestart with another well bore trajectory.

For loss of drilling fluid while drilling, the problem is often solvedby placing some cement slurry at the bottom of well and squeezing partof it in the formation. However, the resulting treatment is often notvery deep and when the drilling is reinitiated across the cement plug,the well bore can enter virgin formation again, and losses oftenrestart.

The problem of narrow pressure window is often difficult to solve: thereis limited freedom to adjust mud density while avoiding formationfracturing or influx of formation fluid in the well-bore. Often a casinghas to be installed to isolate that formation.

It is an object of the invention to provide constructions techniquesthat are alternatives to these treatments of methods and which canpotentially overcome some or all of the problems. The invention is basedon the use of lateral boreholes, i.e. secondary boreholes that aredrilled from a main borehole. Laterals have been previously proposed forvarious uses, in particular for providing improved contact with theformation.

DISCLOSURE OF THE INVENTION

One aspect of this invention provides a method of constructing a well,comprising:

drilling a main borehole extending from the surface through one or moreunderground formations; and

drilling a plurality of lateral boreholes extending from the mainborehole into surrounding formations;

wherein the lateral boreholes are substantially shorter and of smallerdiameter than the main borehole; andwherein each lateral borehole is separated from its neighbouring lateralboreholes by a relatively short distance.

The lateral boreholes preferably extend 5-30 metres from the mainborehole and have a diameter in the range 3.8-10 cm.

The lateral boreholes are typically drilled at an axial spacing of lessthan a few metres in the main borehole and more than one lateralborehole can be drilled at the same depth in the main borehole.

In one preferred embodiment, the lateral boreholes are drilled with atrajectory that deviates from the main borehole by less than 10°. Inanother, the lateral boreholes can extend essentially perpendicular tothe main borehole.

In certain cases, it can be preferable to drill the lateral boreholeswith trajectories that extend in a plane that does not contain the mainborehole. The lateral boreholes can have an S-shape or spiral around themain borehole.

A preferred use of the method comprises drilling the lateral boreholesso as to extend through a region of modified formation propertiessurrounding the main borehole, such as skin or drilling damage, into aregion beyond which has substantially bulk formation properties.

The lateral boreholes can be filled with gelled fluid after drilling soas to prevent contamination of the lateral boreholes with fluids fromthe main borehole. The method can also include breaking the gel of thefluid in the lateral boreholes so as to obtain access to the interior ofthe lateral boreholes.

Another embodiment of the method comprises substantially filling thewhole of the lateral boreholes with gravel. Preferably, the gravel isstabilised at the region of the lateral boreholes close to the mainborehole so as to prevent gravel from passing into the main borehole.

Each lateral borehole can be filled with gelled fluid or gravelimmediately after it has been drilled and before another lateralborehole is drilled or one after the other following drilling of all ofthe lateral boreholes.

The main borehole can be completed in the region from which the lateralboreholes extend by means of a gravel pack and screen, an expandablescreen, a slotted liner or cemented casing.

Methods according to the invention can also further comprise pumping aformation treatment fluid through the lateral boreholes so as to modifythe formation properties near the well. The treatment fluid can bepumped into the formation to modify its permeability to restrict flow ofwater or gas into the well, or to stabilise its mechanical propertiesduring the drilling process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show simplified views of wells drilled in accordance withthe invention;

FIGS. 3 and 4 show different forms of lateral well;

FIG. 5 shows treatment and improved contact with of a formation inaccordance with one embodiment of the invention;

FIG. 6 shows completion of a well in accordance with another embodimentof the invention;

FIGS. 7-9 show construction of a horizontal well in accordance with anembodiment of the invention;

FIGS. 10 and 11 show another embodiment of construction of a horizontalwell in accordance with the invention; and

FIGS. 12 and 13 show treatment of a formation during drilling using atechnique according to the invention.

MODE(S) FOR CARRYING OUT THE INVENTION

This invention is based on the concept of multiple small lateralsdrilled from parent, wells or boreholes. This invention also includestreatments which can be performed in and from the small lateral to adaptor correct the performance of the main well, the formation properties,the formation fluids and the change of porosity and permeability of theformation. The laterals are typically 5 to 30 m long (compared to mainborehole depths of several thousand metres), and of 1.5 to 4 inch(3.8-10 cm) diameter (compared to main borehole diameters typically inthe range 20-40 cm). The trajectory of these laterals can be eithernearly parallel to main well, with deviations below 10°) or as deviatedas possible (perpendicular) from the main well. The distances betweensuccessive lateral junctions to the parent well can be fairly small:that axial spacing could be as close as zero (i.e. more than one lateralborehole at the same depth) with the lateral boreholes at differentazimuths. Several laterals can be drilled for every meter of main well(when rock strength is not a limitation). The laterals can be S-shapedor a spiral around the main well in certain cases.

The new treatments which are provided by this invention are based onfluid or slurry placement techniques in the lateral or in matrixtreatment from the laterals. For example:

Filling the small lateral with gelled fluid, to avoid pollution of thelaterals from the main borehole during future operations.

Filling the lateral with gravel for the purpose of sand management. Asmultiple laterals are treated, the global results can provideimprovement over conventional “Pack&Frac” or “sand management” in longhorizontal drains.

Matrix treatments via the laterals to solve drilling problems, e.g.control of drilling fluid loss, management of kicks and influxes, rockstrengthening, etc.

Matrix treatments to solve production problems, such as water arrival inhorizontal wells, or re-development of contact with the reservoir atproper depth when in valleys or hills of a horizontal well.

Improvement of productivity (PI), by passing skin layer and limitingpressure draw-down and its risk of PVT transition.

FIGS. 1 and 2 show a main borehole 10 can be provided with multiplelateral boreholes 12 closely spaced together. In many cases, aconventional curved trajectory can be used for the laterals 12 (seeFIG. 1) giving a fishbone arrangement when seen in two dimensions. Itmay also be useful for the laterals 12 to extend directly away from themain borehole 10 as is shown in FIG. 2. The laterals may be placed atdifferent angle with the main well.

For some formation treatments, other well trajectories may be adoptedsuch as those shown in FIGS. 3 and 4. FIG. 3 shows S-shaped laterals 12can insure a contact with the reservoir which could be more “parallel”to the main borehole 10. This can be advantageous for example fortreatments near a horizontal main borehole.

The spiral shape lateral 12 shown in FIG. 4 can be advantageous foraxisymmetrical treatment around the main borehole 10. This may bebeneficial when applying treatments in the near-well bore region.

The well production can be increased via a larger contact surface to thereservoir. Furthermore the contact can be insured at a fair distanceform the main well bore, so that the pressure drawdown due to theconcentric flow is limited. This can be particularly useful where highskin is present and where the formation fluid is heavy oil. FIG. 5 showssuch an implementation. In this case, the small laterals 12 are drilledfrom the parent well 10 a sufficient distance into the formation 14 soas to pass through the skin 16 around the parent well 10 with highpressure loss characteristics and into formation 14 displaying proper,bulk properties.

The well production can also be increased with the help of smalllaterals in the situation of horizontal hole drilled for production ofreservoir formed by lenses separated by insulating shale. Each littlelaterals may contact multiple lenses increasing drastically therecovery.

The well production can also be increased with the help of smalllaterals when producing highly fractured reservoir via a single mainquasi vertical well: the laterals may be drilled in direction nearlyperpendicular to the factures to insure more interconnections.

In one embodiment of the invention, the laterals are filled with gelledfluid after its drilling. Thanks to this fluid, the lateral will not bepolluted by other fluids such as drilling mud and/or cement slurry inthe parent well, the lateral staying clean until needed for later use.The gelled fluid can be placed in the lateral as a fluid pill by thetool which has been used to drill that lateral before it is moved toanother location. For example, the main well can be drilled to targetdepth (TD); then multiple small laterals are drilled and filled withgel. Then casing and cementing isolation can be performed for the mainwell. Finally high density perforation can be performed to connect thelaterals to the main well to insure better drainage of the reservoir.

Breaking of the gelled fluid to allow clean-up of the laterals can bedue to time. Other methods can also be used, such as injection of anappropriate breaker fluid into the lateral, similar to techniques usedfor breaking gelled fracturing fluid.

In one embodiment of the invention, the whole volume of the lateral isfilled with gravel, such as is used for gravel packing. This isdifferent to the conventional gravel packing, where the centre of thewell is kept open by the screen. The produced fluid enters the lateraland then flows to the main well via the packing in the lateral. Thispacking preferably has permeability properties similar to fracturingwith proppant. However, in this application, the gravel is not submittedto the high closing stresses as are present in fractures. This givesmore freedom to select the gravel. The main properties of interest are:

Screening against the flow of formation sand.

High axial permeability.

Gravel stability at the top of the lateral.

It is preferred that gravel used to pack the lateral should not beentrained into the parent well. To achieved this effect, the upper partof the lateral may be packed with gravel containing fibres, roughgravel, piece of cloth, sand covered with resin, etc. to stabilise thepack. This may only be necessary for the last few meters of the lateralnear the junction.

For this treatment, the laterals may be advantageously steered away fromthe main well (as perpendicular as possible) to reduce the pressuredraw-down in the reservoir.

The packing of each well can be performed when the drilling system usedto drill the laterals is still in place. However in this situation, thecirculation of the small slurry volume to the bottom of the main wellfor packing may require a long time as the main well can be relativelydeep. To avoid the repeated loss of time for individual treatment ofeach of the laterals, it may be preferred to place the packing in alllaterals in one step. For this method, it is necessary to re-enter inthe laterals. An appropriate tool can be used to facilitate thisre-entry (such as for operation with coiled tubing in multi-lateralwells). During lateral packing, the gravel slurry is pumped slowlythrough the tip of a pipe in the lateral while the pipe is pulled backslowly. Proper coordination between flow rate and pulling of the pipe isneeded to insure full packing of the small lateral.

In the main well, the production interval can be protected in a numberof ways as are described below.

Open-hole gravel packing and screens may be used. This corresponds tothe situation of “Pack&Frac”. It gives good PI contact to the reservoirwith low production velocity to avoid damage in the packing.

Alternatively, expandable screens may also be used giving a wider borefor flow in the well.

A slotted liner can also be used in combination with the small packedlaterals as is shown in FIG. 6. The liner 18 ensures that the main well10 does not collapse, for example because of the presence of anunconsolidated formation 20. In such a case, it may be necessary toensure that the production only occurs via the small laterals 12.Without additional care, some production may be achieved directly viathe surface of the main well 10 potentially leading to sand productionwith its associated risk. For the survival of the main well 10, atreatment of the near-bore 22 is performed to stabilize the formation inthe vicinity of the main well-bore 10. This matrix treatment can beperformed just after drilling the main well (before any laterals aredrilled). An appropriate treatment fluid is placed at the desiredinterval in the main well 10. It is then injected in the formation 22over the interval of the well, to stabilize the rock (increase itsstrength to erosion) or to seal it over a short depth (e.g. 1 foot/30cm) to insure that production via the sand face is blocked (theproduction 24 will be via the laterals 12 which are treated near thejunction 26 with the main well 10 to prevent production of the gravel).

Cemented casing in the main well. In this case, it is probably better todrill the small lateral after the installation of the casing.

The use of multiple small laterals without gravel packing may be anadequate solution for production where sanding normally occurs. It maynot be necessary to pack the small lateral for the following reasons:

reduction of the pressure drawdown thanks to a increase contact with thereservoir;

lower fluid velocity in the vicinity of the small laterals; and/or

high stability of the small well-bore thanks to the small diameter.

In horizontal wells, the well trajectory is not always perfectlyhorizontal or parallel to the water table (which lies below theoil-bearing zone). In some intervals, the distance between the well andthe water table may be smaller than others. Water coning can appearquickly in this positions when open-hole production or slotted linersare used, or even with dense perforation schemes.

This problem can be addressed by the use of small laterals in accordancewith the invention as is shown in FIGS. 7-9. Multiple small laterals 112are drilled down from the parent well 110 towards the water table 114. Amatrix treatment is then performed via the small laterals 112 to injectsealing product into the pores of the formation. The objective of theseinjections is to cerate a non-permeable disk 116 between the water table114 and the well 110. These disks 116 would then form an impermeablelayer which would limit the move upwards of the water.

These treatments are typically performed very early in the life of thewell, for example just after drilling when the proximity of the watertable is detected. However, treatment can also be performed later whenproduction is performed in open-hole.

For this application, S-shape laterals 118 may be preferred as theywould insure a better placement of the fluid in the formation as isshown in FIGS. 10 and 11.

With cased-hole, the lateral drilling is slightly more complex due tothe opening of the window in the casing.

Techniques according to the invention can be used for water productionmanagement in horizontal wells. For example, the main (horizontal) wellcan be drilled at the top of the reservoir (or even above the reservoir)and multiple small laterals are then be drilled downwards to ensure goodconnection with the reservoir. These small lateral can be gravel packed(over their whole section) as explained above. The packing contains“conventional particles” such as those used in conventional packing or“pack& frac”, but also contains materials which swell when in contactwith water. This means that the lateral length in contact with the water(water table or water coning) would let the water be produced for alimited period. Then the swelling material blocks the permeability ofthe drain over the water wet interval. This ensures an automaticlimitation of the water entry in the laterals (and in the main well).

In horizontal wells, the well may locally be close to the interface withthe gas cap lying over the oil-bearing zones. In this case, there can bea risk of gas entering in the well which could then reduce the totalwell production capacity, as the gas may limit the well section involvedwith liquid production. Furthermore, the gas production into the mainwell may also cause rapid pressure reduction in the reservoir such thatthe natural flow will be reduced. The gas production in the peaks of thewell trajectory is similar to the water production in the troughs andsimilar treatment can be applied to limit the gas coning effect.

As shown in the real field situations, horizontal wells may have troughsthat are too close to the water table for proper connection to theoil-bearing part of the reservoir. It may be beneficial to apply theabove technique (such as described in FIG. 11) locally in the throughsof the horizontal well to retract the local water coning effect.

In another embodiment of the invention, small laterals are drilledupwards to ensure drainage from higher zone of the oil-bearingformation.

Techniques according to the can also be used to address drillingproblems. These include:

High (total) loss of drilling fluid (including the case of lostcirculation): this is often due to low pressure formation with highpermeability or highly fractured layers;

Well-bore influx from high pressure formation. In some cases, it may bedifficult to increase mud density to reach the proper pressureequilibrium for the high pressure zones without fracturing otherformations;

Rupture of formation with inadequate mechanical properties. The rock mayfail under “tensile” load (commonly called fracturing): one normaltreatment is to reduce the mud density but this can lead to problemswith well bore collapse as the well-bore hoop stress is too high (thisis typical in horizontal wells); another normal treatment is to increasemud density but again, the mud density adjustment may be limited due tolimitations by other formations.

It is often difficult to find the correct mud density to address allpotential drilling problems and allow safe, effective drilling tocontinue. The ultimate solution is often to install a casing string toisolate the problem formation. However, casing is expensive and, thetelescopic effect of successive casing strings makes it difficult toprovide the correct well-bore size in front of the reservoir. In theworst case, the well may have to be abandoned as the drain is too smallin diameter with too low productivity.

This invention allows combating of the problems in the criticalformation in different ways. One embodiment of the invention involvesdrilling multiple small laterals 120 at a small distance from the mainwell bore 122 (see FIGS. 12 and 13). In this application the laterals120 are only slightly deviated form the main well 122 (e.g. 5°). Howeverseveral laterals are drilled at the same depth at different azimuths.Spiral laterals (such as are described above in relation to FIG. 4) canalso allow the same result to be achieved. The small laterals 120 arebeing used to inject products into the formation 124 and seal or modifythe formation strength compared to that of the untreated formation 126.Thus the problem formation 126 can be isolated from the normal formation128 and allow further drilling to continue.

Different types of fluid can be injected (squeezed) into the formation,such as:

fine cement slurries (such as SqueezeCrete of Schlumberger) to block thepore and increase the rock strength;

polymers which flow in the rock pores and then solidify (while blockingflow and increasing rock strength); and

gels to block the pores against flow and then break down after theproper triggering mechanism, including time (this approach may beinteresting if the initial porosity and permeability need to berecovered after drilling has finished).

These treatments are typically performed as soon as the criticalformation has being drilled.

Other changes within the scope of the invention will be apparent.

1-21. (canceled)
 22. A method of constructing a well, comprising:drilling a main borehole extending from the surface through one or moreunderground formations; and drilling a plurality of lateral boreholesextending from the main borehole into surrounding formations; whereinthe lateral boreholes are substantially shorter and of smaller diameterthan the main borehole; and wherein each lateral borehole is separatedfrom its neighboring lateral boreholes by a relatively short distance.23. A method as claimed in claim 22, comprising drilling the lateralboreholes so as to extend 5-60 meters from the main borehole.
 24. Amethod as claimed in claim 22, comprising drilling the lateral boreholesso as to have a diameter in the range 3.8-10 cm.
 25. A method as claimedin claim 22, comprising drilling more the lateral boreholes at an axialspacing of less than a few meters in the main borehole.
 26. A method asclaimed in claim 25, comprising drilling more than one lateral boreholeat the same depth in the main borehole.
 27. The method of claim 22,comprising drilling the lateral boreholes with a trajectory thatdeviates from the main borehole by less than 10 degrees.
 28. The methodof claim 22, further comprising drilling the lateral boreholes withtrajectories that extend in a plane that does not contain the mainborehole.
 29. The method of claim 22, comprising drilling the lateralboreholes so as to have an S-shape or so as to spiral around the mainborehole.
 30. The method of claim 22, comprising drilling the lateralboreholes so as to extend through a region of modified formationproperties surrounding the main borehole into a region beyond which hassubstantially bulk formation properties.
 31. The method of claim 22,further comprising filling the lateral boreholes with gelled fluid afterdrilling so as to prevent contamination of the lateral boreholes withfluids from the main borehole.
 32. The method of claim 31, furthercomprising breaking the gel of the fluid in the lateral boreholes so asto obtain access to the interior of the lateral boreholes.
 33. Themethod of claim 22, comprising substantially filling the whole of thelateral boreholes with gravel.
 34. A method as claimed in claim 33,further comprising stabilizing the gravel at the region of the lateralboreholes close to the main borehole so as to prevent gravel frompassing into the main borehole.
 35. The method of claim 31, furthercomprising filling each lateral borehole with gelled fluid or gravelimmediately after it has been drilled and before another lateralborehole is drilled.
 36. The method of claim 31, further comprisingfilling all of the lateral boreholes with gelled fluid or gravel oneafter the other following drilling of all of the lateral boreholes. 37.The method of claim 22, further comprising completing the main boreholein the region from which the lateral boreholes extend by means of agravel pack and screen, an expandable screen, a slotted liner orcemented casing.
 38. The method of claim 22, further comprising pumpinga formation treatment fluid through the lateral boreholes so as tomodify the formation properties near the well.
 39. A method as claimedin claim 38, comprising pumping a treatment fluid into the formation tomodify its permeability to restrict flow of water or gas into the well.40. A method as claimed in claim 38, comprising pumping a treatmentfluid into the formation to stabilize its mechanical properties duringthe drilling process.
 41. A method as claimed in claim 38, comprising asets of small laterals drilled from a main horizontal well, such asformation treatment fluid can be injected from the end part of thelaterals into the formation to create an “extended barrier” againstwater or gas coning towards the main horizontal well.